1. Field of the Invention
The present invention relates to fuel cells and in particular, to hydrogen and oxygen fuel cells, and more specifically relates to the electrode structure of such cells.
2. Discussion of the Related Art
As illustrated in FIG. 1, a hydrogen-oxygen fuel cell comprises a layer or a sheet of an electrolyte 1 sandwiched between two catalyst layers or sheets 3 and 4 coated with conductive layers 6 and 7 intended for the contacting. The upper cell surface is in contact with oxygen, for example, air, and the lower cell surface is in contact with hydrogen.
Under such conditions, when the cell is connected to a load 8, a positive voltage appears on the side of the upper surface or cathode and a negative voltage appears on the side of the lower surface or anode and a current flows through the load. On the anode side, the catalyst transforms gaseous hydrogen molecules into two protons and two electrons, the protons flow from the anode catalyst layer, through the electrolyte layer to the cathode catalyst layer where reaction 2H++½O2+2e−→H2O occurs, with both electrons flowing through the load.
Presently, electrolyte 1 is Nafion and catalyst 3, 4 is a mixture of platinum carbon, for example comprising a few percents of platinum, and preferably contains an amount of Nafion, for example, from 20 to 40%.
Conductors 6 and 7 for example are very thin gold layers to be both conductive and permeable to hydrogen or oxygen. Conductors 6 and 7 may also be formed of gold grids. In the case where conductors 6 and 7 are very thin gold layers, for example, of a thickness below 1 μm, to be sufficiently permeable to gas, they are poorly conductive since the catalyst layers are porous layers and the gold layers do not form even planar surfaces, but deposit discontinuously by penetrating into the catalyst pores. A problem of contact resistance (or current collection) thus arises. In the case where conductors 6 and 7 form a gold grid with meshes of a thickness from 2 to 3 μm, an obvious disadvantage is that, under the solid portions of the grid, the gas cannot react with the underlying catalyst.
Another parameter which adversely affects a satisfactory current collection is that the material of the catalyst, as indicated previously, preferably is a mixture of platinum carbon and electrolyte (for example, Nafion). The presence of Nafion in the catalyst layers enhances a good operation of the cell but since Nafion is a good proton conductor but a poor electron conductor, this increases the internal resistance of the cell. As a result, the proportion of Nafion has to be decreased in the catalyst, which adversely affects the efficiency of this catalyst. Indeed, when the amount of Nafion in the electrode is decreased, the proton conduction across the electrode bulk, and thus the amount of exchanged protons, is decreased.
FIG. 2 shows an embodiment of a hydrogen and oxygen fuel cell. The cell is formed from a support made of a portion of a silicon wafer 10, preferably thinned down in the portion where it supports the actual cell. In this thinned-down portion, openings 11 letting through oxygen are made in silicon wafer 10. It should be understood that, generally, all the wafer surfaces are coated with an insulator formed at least of native silicon oxide. An active cell area is delimited, substantially above the thinned-down region comprising the openings, by an insulator layer 12, for example, a silicon oxide layer. Inside of the opening formed in layer 12, a catalyst layer 3 is formed by any process, for example by inkjet deposition. An electrolyte 1, such as a Nafion layer, is, for example, spun on and covered with a second catalyst layer 4. Previously, a conductor 6, such as a gold anode layer, for example having a thickness on the order of one micrometer, has been formed on the surface of a support such as silicon wafer 10. Conductor 7 may be a gold cathode layer for example having a thickness on the order of one half micrometer formed on the upper surface of the structure. The foregoing is an example only of a possible embodiment of a hydrogen-oxygen fuel cell, this embodiment having the advantage of using technologies well known in silicon-based component manufacturing for the support. In such a structure, the contact between conductor 6 corresponding to a conductive anode layer and catalyst layer 3 is not a major issue but the quality of the contact between conductor 7 corresponding to a thin gold cathode layer and catalyst layer 4 is difficult to achieve if, as indicated previously, both a good collection of charge carriers and a good “transparency” of the layers for a proper diffusion of the oxygen in catalyst layer 4 are desired to be obtained. Currently, the useful surface area of the fuel cell ranges from 1 to 3 cm2.